A patient presented with the clinical features of a brain stem infarct. MRI confirmed changes of infarction but also revealed an unusual lesion in the left cerebellopontine angle cistern. The evolution of this lesion on MRI, together with angiographic changes, indicated a dissecting aneurysm of the left anterior inferior cerebellar artery, a previously unreported phenomenon. The presentation, causes and diagnosis of intracranial dissection are discussed.